1. Field of the Invention
The invention relates to a transmission system comprising a transmitter for transmitting a transmit signal formatted in blocks coming from symbols of a constellation, the same block containing the same information signal present a first and a second time, and a receiver for receiving said signal.
The invention likewise relates to a receiver used in such a transmission system.
2. Discussion of the Related Art
In a general manner the invention relates to signals formatted in blocks for which said blocks may have a great complexity due to redundant information. They may be, for example, signals obtained from orthogonal N-frequency division multiplexing which consists of splitting up a transmit information signal by distributing same over a large number of low-rate elementary channels. One thus transforms a highly selective wideband channel into a large number of non-selective elementary channels. As the arrangement forms a wideband channel, it is not very likely that fading during transmission will simultaneously affect the whole channel. This technique specifically makes it possible to reduce intersymbol interference.
One frequency thus corresponds to each elementary channel, all the frequencies together being a symmetrically distributed around a central carrier frequency. As it is hard to accept the use of selective filters at the receiver end, one prefers tolerating an overlap of the spectra but then requirements are imposed with respect to orthogonality between the frequencies to eliminate intersymbol interference at the sampling instants. The whole spectrum of an OFDM signal thus tends towards a rectangular spectrum.
At the receiver end the received signals are to be demodulated and then decoded to restore the original information. For this demodulation is used a local oscillator whose frequency is to be locked on to the transmit frequency.
The OFDM signal is subdivided into frames formed by blocks of which certain blocks are service blocks and other blocks are data blocks. To avoid intersymbol interference, each block contains redundant information signals. Any block is formed at all the OFDM frequencies the device uses, the frequencies being modulated by transmit symbols coming from a coded modulation, for example, a digital PSK or QAM modulation. At the transmitter end, the symbols are coded with a certain timing which is to be found back at the receiver end, so that they can be decoded correctly. The invention proposes to find back at the receiver end the timing used at the transmitter end.
FR 2 639 495 is known which describes a digital data transmission method which employs an OFDM technique. The method of synchronization recovery is carried out with a frame while use is made of two specific blocks which are a zero block at the beginning of the frame and a wobulation block. This document puts forward the importance it presents of not synchronizing a timing clock with the binary level as in previous techniques. Said document thus works with binary data acquisition windows which it considers to have a sufficiently high precision for decoding the binary data.
But when a synchronization is effected once per frame, a frame having, for example, a duration of 20 ms, the precision of the synchronization remains insufficient.
Thus, it is an object of the invention to provide a clock synchronization with a better precision than in the prior art for a signal formatted in blocks which themselves contain redundant information signals.